3.4 Equal friction method

• This method of sizing is used for supply, exhaust and return air duct system and employs the same friction lose per foot of length for the entire system.

• The equal friction method is superior to velocity reduction since it requires less balancing for symetrical layouts.

• If a design has a mixture of short and long runs, the shortest run requires considerable dampering.

• Such a system is difficult to balance since the equal friction method makes no provision for equalizing pressure drops in branches or for providing the same static pressure behind each air terminal.

• This method establishes an equal friction drop for each 100 ft of duct throughout the duct system.

• Ducts are sized for a constant pressure loss per unit length

• Using 0.1 inWG / 100 ft as recommended source.

Step 1Select starting velocity at fan discharge duct. Selected velocity should not be more than recommended velocity.

Equal Friction Method Procedure

Step 2Obtain circular duct diameter and friction loss from Circular Duct Friction Chart by using air velocity and airflow.

Step 3Select equivalent rectangular duct size. Using the circular duct diameter andequivalent area based on available space.

Step 4Select the next duct section by maintaining the same friction loss rate which has been obtained in previous duct section. Cross check the velocity should not exceed the recommended velocity. Repeat the method for consecutive section

Equivalent Lengths of Round Duct Elbow :-

Regain

WhereVd = duct velocityVf = fan outlet velocity

Loss is in.WG

Formula

Example:• Given a simple ducting layout,

calculate using duct calculator the values of Qv, V, rectangular duct and round duct size using equal friction method.

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